1. Field of the Invention
The present invention concerns a component that can in particular constitute a semiconductor optical amplifier. An amplifier of this kind has an active structure consisting of an undoped active layer or a plurality of such layers stacked in a vertical direction. This structure simultaneously constitutes a waveguide guiding light waves in a longitudinal direction. It is placed between two injection layers, a p-doped layer and an n-doped layer. A pump current flows in the vertical direction to inject n and p type carriers into the active layer(s) from the respective injection layers. The presence of the two types of carrier is necessary for the active layers to be able to amplify light passing through them by virtue of recombination of carriers of one type with those of the other type.
2. Description of the Prior Art
Amplifiers of the above kind are typically used at the nodes of fiber optic telecommunication networks. The information transmission capacity of a network of the above kind is increased by wavelength-division multiplexing, which enables a plurality of carrier waves to be transmitted on the same optical fiber. Within a node of the network, the same amplifier is then used to confer an appropriate optical power on all the waves. The wider the spectral range that these waves occupy, or could occupy, the greater the capacity of the network. However, the amplifier can impart this power to these waves only if their wavelengths are within a gain band of the amplifier. This is why the need for a wider gain band has often been felt.
Such widening has been obtained in a first prior art amplifier in the form of a waveguide amplifier constituting an amplifying part of a tunable laser oscillator. The oscillator is described in U.S. Pat. No. 4,680,769 (Miller). Its amplifying part includes two of said active structures in succession in the longitudinal direction and each consisting of an active layer, i.e. the light passes through the two structures in succession and two pump currents are supplied for the respective structures.
The gain band of the above amplifier is widened because the layers constituting the two active structures are made of two different materials. This difference in composition leads to a difference between the respective energy gaps between the valency band and the conduction band in the two layers. These bands are those in which the energy levels that can be occupied by the electrons are situated. For the active material these energy gaps define a characteristic wavelength that constitutes an upper limit on the gain band and thereby defines the position of that band within the spectrum of wavelengths. As a result the gain bands of the two active structures are offset relative to each other. They partly overlap each other, however. The wavelength emitted by the oscillator can be tuned by virtue of the fact that, in each of the two structures, the gains of the structure for wavelengths included in the gain band of the structure are controlled by the pump current. The two pump currents are then chosen to obtain a maximum gain for a chosen wavelength in the gap between the center wavelengths of the two gain bands specific to the two active structures.
If it had to be used as a widened gain band amplifier, this first prior art amplifier would have the drawback of requiring adjustment of two pump currents.
It is now widely known that the gain band of a semiconductor waveguide amplifier is greater if the active structure of the amplifier comprises quantum wells than if it comprises a bulk active layer like that of each of the two active structures of the first prior art amplifier. A bulk layer of the above kind is a layer formed from a bulk material. The wells of a quantum well structure take the form of stacked thin layers separated from each other by barriers whose thicknesses are comparable with those of the wells. With the aim of confining the waveguide functions of the charge carriers in each of the above wells, the material of the well is chosen to have an energy gap much less than that of the barriers. The difference between the two gaps is at least 100 meV and typically around 250 or 300 meV. The thickness of each such well is typically in the order of 10 nm while that of a bulk layer is typically in the order of 100 nm. This is why, to obtain the same total thickness of active material, an active structure with quantum wells typically comprises around ten wells. The gain band of the resulting structure is widened because the small thickness of the wells increases the density of the energy levels that can be occupied by the electrons in the wells. It is nevertheless desirable to increase further the gain bandwidth of an amplifier of the above kind.
This widening is achieved in a second prior art amplifier which takes the form of a semiconductor waveguide amplifier whose active structure comprises quantum wells. The second prior art amplifier is described in a paper given to the Topical Meeting on Quantum Optoelectronics, Salt Lake City, USA, March 1991, O.S.A. 1991 Tech. Digest Ser . . . , "Broadband GaAs/Al.sub.x Ga.sub.1-x As Multi-Quantum Well LED", A. J. Moseley, D. J. Robbins, C. Meaton, R. M. Ash, R. Nicklin, P. Bromley, R. R. Bradley, A. C. Carter, C. S. Hong and L. Figueroa. The widening of the gain band results from the fact that the quantum wells of the active structure comprise active materials having different compositions from well to well. As previously explained, because of such differences in composition the successive wells have gain bands offset relative to each other. The offset between two successive wells is sufficiently small for the respective gain bands of the two wells to overlap partly, which results in a substantially continuous gain band for the active structure. This band can be very wide because it includes the respective gain bands of all the wells. The second prior art amplifier has the following drawbacks:
It is complex to make. PA1 Obtaining the required gain in each of the gain bands of the various active layers requires the pump current to create a sufficient positive and negative charge carrier density in each layer. This requirement is easily met for negative charge carriers (electrons) because their high mobility means that they can quickly reach the quantum wells at the greatest distance from the n type injection layer from which they were injected into the active structure. Unfortunately, the same cannot be said for the positive charge carriers (holes), which are well known to be much less mobile. Large number of holes could disappear through recombination in the wells nearest the p type injection layer from which they originate and consequently not be injected in sufficient numbers into the wells at the greatest distances from that layer. This could lead to insufficient gain or excessively high instability of the gain in the gain bands of the active layers nearest the n type injection layer. To prevent this lack of uniformity of injection of holes making the gain of certain wells too low or excessively unstable, all the quantum wells and all the barriers of the second prior art laser are heavily p-doped, which artificially increases the density of holes at all points. This heavy doping has the drawback of further complicating the production of the amplifier and above all of increasing the absorption of light in the active structure. Any such loss of light must be compensated by increasing the intensity of the pump current, which leads to a corresponding increase in the thermal power to be evacuated.
One particular aim of the present invention is to enable simple production of a semiconductor optical amplifier with a widened gain band and the invention consists in particular in an amplifier of this kind.